[PATCH] PR libstdc++/87704 fix unique_ptr(nullptr_t) constructors

[PATCH] PR libstdc++/87704 fix unique_ptr(nullptr_t) constructors

Using a delegating constructor to implement these constructors means
that they instantiate the destructor, which requires the element_type to
be complete. In C++11 and C++14 they were specified to be delegating,
but that was changed as part of LWG 2801 so in C++17 they don't require
a complete type (as was intended all along).